Emissive flat-panel display devices are widely used in conjunction with computing devices and in particular with portable devices. These displays are often used in public areas with significant ambient illumination.
Organic light emitting diodes (OLED) have many advantages in a flat-panel display device and are useful in optical systems. U.S. Pat. No. 6,384,529 issued May 7, 2002 to Tang et al. shows an OLED color display that includes an array of OLED light emitting elements (pixels). Light is emitted from a pixel when a current is passed through an organic material, the frequency of the light depending on the nature of the organic material that is used. The organic materials are placed upon a substrate between electrodes, with an encapsulating cover layer or plate. In such a display, light can be emitted through the substrate (a bottom emitter) or through the encapsulating cover (a top emitter), or both. The emitted light is Lambertian, that is it is emitted equally in every direction. Because OLED devices typically employ a reflective back electrode, ambient light is typically reflected and the contrast of the display is of great concern. Similarly, inorganic LED devices, for example, comprising quantum dots in a polycrystalline semiconductor matrix, located between a reflective electrode and a transparent or semi-transparent electrode, suffer from ambient contrast concerns.
The ambient contrast ratio of a display is a performance factor that requires high-light emission combined with a low reflectivity. Because LED devices typically employ a reflective back electrode, providing a good ambient contrast ratio can be problematic. It is known to use a circular polarizer affixed to the surface of the display so that light incident on the display is absorbed by the polarizer, while light emitted by the display is not. For example, WO 02/10845 A2 entitled, “High Durability Circular Polarizer for use with Emissive Displays” published Feb. 7, 2002 describes a high durability circular polarizer including an unprotected K-type polarizer and a quarter-wavelength retarder. This circular polarizer is designed for use with an emissive display module such as an organic light emitting diode or a plasma display device. However, while effective in reducing reflections in high-ambient light conditions, for example outdoors on sunny days, such polarizers do not always provide adequate contrast and they absorb more than half of the emitted light.
Other methods of improving contrast and readability in a display rely on increased light output. For example, optical cavity structures are known to increase the light emitted from an OLED device structure. Such optical cavity structures are also known as microcavities or optical microcavities (and are used interchangeably herein) when formed in thin films and rely on a patterned deposition of different color light-emitting organic materials over a substrate between a reflective electrode and a semi-transparent electrode. Light emitters having different colors are formed within an optical cavity tuned to a desired peak wavelength of light corresponding to the color of light emitted by the patterned organic materials. U.S. Pat. No. 6,680,570 describes an organic light-emitting device with improved color control employing spacer layers to form an optical cavity. FIG. 7 illustrates such a prior-art, active-matrix, bottom-emitting optical cavity device employing a substrate 10 with active-matrix thin-film components 30, planarization structures 32 and 34, and a semitransparent electrode 12. Patterned organic materials 14R, 14G, and 14B corresponding to red, green, and blue light emission are deposited in a light-emitting layer 14. Optical spacers 26R, 26G, and 26B are employed to form optical cavities 60, 62, and 64 tuned to the desired peak wavelengths of red, green, and blue light, respectively to emit red light 80, green light 82, and blue light 84. A cover 20 can be employed to protect and encapsulate the device. While such designs are useful, they require a patterned organic material deposition technology that is difficult to scale to large substrates. Moreover, optical cavity devices typically suffer from an unacceptable angular dependence on color. It is also known to employ a color filter with an optical cavity structure, for example as taught in U.S. Pat. No. 7,189,238. However, while useful, such an approach does not improve the manufacturability of the device and provides inadequate contrast ratio under some illumination conditions.
U.S. Pat. No. 5,554,911 entitled “Light-emitting elements” describes a multi-color light-emitting element having at least two optical cavity structures with respectively different optical lengths determining their emission wavelengths. Each optical cavity structure includes an organic material as a light-emitting region, which may be a single film of uniform thickness in the element. U.S. Pat. No. 6,861,800 entitled, “Tuned microcavity color OLED display” describes a microcavity OLED device having an array of pixels divided into at least two different color pixel sets, each color pixel set emitting a different predetermined color light over a common substrate, wherein each pixel in the array includes a metallic bottom-electrode layer disposed over the substrate and a metallic electrode layer spaced from the metallic bottom-electrode layer. The material for the semitransparent metallic electrode layer includes Ag, Au, or alloys thereof. The thickness of the semitransparent metallic electrode layer, the combined thickness of the organic layers and the transparent conductive phase-layer, and also the placement of the light-emitting layer are selected so that each pixel in the display forms a tuned microcavity OLED device having emission output efficiency above that of a comparable OLED device without the microcavity. U.S. Pat. No. 5,949,187 describes an OLED with a first microcavity including a first transparent spacer and a first mirror stack positioned on the first spacer to reflect light back into the OLED and to define an optical length of the first microcavity. The optical length of the first microcavity being such that light emitted from the first microcavity has a first spectrum. A second microcavity includes a second transparent spacer positioned adjacent the first microcavity and a second mirror stack positioned on the second spacer reflects light toward the first microcavity and defines an optical length of the second microcavity. The optical length of the second microcavity being such that light emitted from the second microcavity has a second spectrum. Additional microcavities can be placed in the structure to further enhance and alter the light spectrum. Such designs, however, may have increased manufacturing costs, lower light output than desired, and reflectance larger than may be desired, as well as significant color change with changes in viewing angle.
US 2006/0066228 A1 entitled, “Reducing or eliminating color change for microcavity OLED devices”, by Antoniadis discloses a microcavity OLED device that minimizes or eliminates color change at different viewing angles. The OLED device can be, for example, an OLED display or an OLED light source used for area illumination. This OLED device includes a multi-layer mirror on a substrate, and each of the layers is comprised of a non-absorbing material. The OLED device also includes a first electrode on the multi-layered first mirror, and the first electrode is substantially transparent. An emissive layer is on the first electrode. A second electrode is on the emissive layer, and the second electrode is substantially reflective and functions as a mirror. The multi-layer mirror and the second electrode form a microcavity. On a front surface of the substrate is a light modulation thin film. The light modulation thin film can be any one of: a cut-off color filter, a band-pass color filter, a brightness enhancing film, a microstructure that attenuates an emission spectrum at an angle at which there is a perceived color change, or a microstructure that redistributes wavelengths so the outputted emission spectrums have the same perceived color. Again such designs may have increased manufacturing costs and reflectance larger than may be desired.
U.S. Pat. No. 7,030,553 entitled “OLED device having microcavity gamut sub-pixels and a within gamut sub-pixel” by Winters et al discloses an example of a prior-art microcavity device. This disclosure describes an OLED device including an array of light emitting pixels, each pixel including sub-pixels having organic layers including at least one emissive layer that produces light and spaced electrodes. There are at least three gamut sub-pixels that produce colors that define a color gamut and at least one sub-pixel that produces light within the color gamut produced by the gamut sub-pixels. At least one of the gamut sub-pixels includes a reflector and a semitransparent reflector, which function to form a microcavity. However, this design employs a patterned semi-transparent electrode that can be difficult to manufacture in a top-emitting format. Moreover, applicants have determined that color purity may be reduced in such a design and reflectance will likely be larger than desired.
One approach to overcoming material deposition problems on large substrates is to employ a single emissive layer, for example a white-light emitter, together with color filters for forming a full-color display, as is taught in U.S. Pat. No. 6,987,355 entitled “Stacked OLED Display having Improved Efficiency” by Cok and commonly assigned herewith. However, the use of color filters substantially reduces the efficiency of the device.
There still remains a need; therefore, for an improved light-emitting structure that overcomes shortcomings in the prior art and that increases the light output and ambient contrast ratio of an LED device.